Scroll devices are well known in the field of vacuum pumps and compressors. In a scroll device, a movable spiral blade orbits with respect to a fixed spiral blade within a housing. The movable spiral blade is connected to an eccentric drive mechanism. The configuration of the scroll blades and their relative motion traps one or more volumes or “pockets” of a gas between the blades and moves the gas through the device. Most applications apply rotary power to pump the gas through the device. Oil-lubricated scroll devices are widely used as refrigerant compressors. Other applications include expanders, which operate in reverse from a compressor, and vacuum pumps. Scroll pumps have not been widely adopted for use as vacuum pumps, mainly because the cost of manufacturing a scroll pump is significantly higher than a comparably-sized, oil-lubricated vane pump. Dry scroll pumps have been used in applications where oil contamination is unacceptable. A high displacement rate scroll pump is described in U.S. Pat. No. 5,616,015, issued Apr. 1, 1997 to Liepert.
A scroll pump includes stationary and orbiting scroll elements, and a drive mechanism. The stationary and orbiting scroll elements each include a scroll plate and a spiral scroll blade extending from the scroll plate. The scroll blades are intermeshed together to define interblade pockets. The drive mechanism produces orbiting motion of the orbiting scroll element relative to the stationary scroll element so as to cause the interblade pockets to move toward the pump outlet.
Scroll pumps typically utilize one or more devices for synchronizing the intermeshed scroll blades. Each synchronizing device is coupled, directly or indirectly, between the stationary and orbiting scroll elements and is required to permit orbiting movement while preventing relative rotation of the scroll elements. In one prior art approach, three crank mechanisms are connected between the orbiting and stationary scroll elements.
U.S. Pat. No. 5,951,268, issued Sep. 14, 1999 to Pottier et al. discloses scroll pumps which utilize a metal bellows for synchronizing the intermeshed scroll blades. The metal bellows surrounds the crankshaft and is connected to the crankshaft on one end and to a stationary wall at the other end. Since a metal bellows has a high resistance to torsional deformation, it can be used to prevent rotation of the orbiting scroll element. However, abnormal torsional loads, which occur during startup and when the pump ingests debris, may overstress and possibly cause failure of the metal bellows.
A scroll pump which utilizes a metal bellows for isolation and which uses crank mechanisms for synchronization is disclosed in U.S. Pat. No. 3,802,809, issued Apr. 9, 1974 to Vulliez. The metal bellows has a fixed connection at both ends and thus may be overstressed in the event of abnormal torsional loads as described above. The disclosed design is torsionally overconstrained, and the crank mechanisms may impose torsional loads on the metal bellows. In addition, the crank mechanisms are located outside the periphery of the scroll blades and add substantially to the size of the pump.
U.S. Pat. No. 4,371,323, issued Feb. 1, 1983 to Fischer et al., discloses a scroll device having at least one parallel motion guide device including an arrangement of leaf springs to ensure torsionally rigid relative movement of two displacement elements. This synchronization method has no axial load carrying capability.
U.S. Pat. No. 4,534,718, issued Aug. 13, 1985 to Blain, discloses scroll apparatus having first and second scrolls which are interconnected by a flexible, circular band located peripherally of the scrolls for synchronization purposes. The circular band can also be used to support the axial load generated by the scrolls. Applicants have found that the circular band disclosed by the Blain patent does not provide satisfactory performance in some applications. For example, the circular band does not rigidly support the axial loads associated with operation of the scroll apparatus. The lateral bending stresses in the circular band are also high, which can limit the life of the synchronization device.
Accordingly, there is a need for improved scroll-type pumping apparatus.